Characterization of a sodium-dependent transport system for butyrobetaine into rat liver plasma membrane vesicles

Lipid Transport in Brown Adipocyte Thermogenesis

Non-shivering thermogenesis is an energy demanding process that primarily occurs in brown and beige adipose tissue. Beyond regulating body temperature, these thermogenic adipocytes regulate systemic glucose and lipid homeostasis. Historically, research on thermogenic adipocytes has focused on glycolytic metabolism due to the discovery of active brown adipose tissue in adult humans through glucose uptake imaging. The importance of lipids in non-shivering thermogenesis has more recently been appreciated. Uptake of circulating lipids into thermogenic adipocytes is necessary for body temperature regulation and whole-body lipid homeostasis. A wide array of circulating lipids contribute to thermogenic potential including free fatty acids, triglycerides, and acylcarnitines. This review will summarize the mechanisms and regulation of lipid uptake into brown adipose tissue including protein-mediated uptake, lipoprotein lipase activity, endocytosis, vesicle packaging, and lipid chaperones. We will also address existing gaps in knowledge for cold induced lipid uptake into thermogenic adipose tissue.

Mass Spectrometry-Based Metabolomic and Proteomic Strategies in Organic Acidemias

Organic acidemias (OAs) are inherited metabolic disorders caused by deficiency of enzymatic activities in the catabolism of amino acids, carbohydrates, or lipids. These disorders result in the accumulation of mono-, di-, or tricarboxylic acids, generally referred to as organic acids. The OA outcomes can involve different organs and/or systems. Some OA disorders are easily managed if promptly diagnosed and treated, whereas, in others cases, such as propionate metabolism-related OAs (propionic acidemia, PA; methylmalonic acidemia, MMA), neither diet, vitamin therapy, nor liver transplantation appears to prevent multiorgan impairment. Here, we review the recent developments in dissecting molecular bases of OAs by using integration of mass spectrometry- (MS-) based metabolomic and proteomic strategies. MS-based techniques have facilitated the rapid and economical evaluation of a broad spectrum of metabolites in various body fluids, also collected in small samples, like dried blood spots. This approach has enabled the timely diagnosis of OAs, thereby facilitating early therapeutic intervention. Besides providing an overview of MS-based approaches most frequently used to study the molecular mechanisms underlying OA pathophysiology, we discuss the principal challenges of metabolomic and proteomic applications to OAs.

Hepatic uptake of gamma-butyrobetaine, a precursor of carnitine biosynthesis, in rats

Gamma-butyrobetaine (GBB) is a precursor in the biosynthesis of carnitine, which plays an important role in the beta-oxidation of fatty acids, and is converted to carnitine by gamma-butyrobetaine dioxygenase (BBD) predominantly in liver. We investigated the molecular mechanism of hepatic uptake of GBB in rat hepatocytes. Cellular localization of rat Octn2 (rOctn2:Slc22A5) was studied by Western blot analysis. Uptake of deuterated GBB (d(3)-GBB) was examined in HEK293 cells expressing rOctn2 (HEK293/rOctn2) and freshly isolated rat hepatocytes. d(3)-GBB was quantified by use of liquid chromatography-tandem mass spectrometry. Western blot analysis demonstrated an expression of OCTN2 protein in hepatic basolateral membrane but not in bile canalicular membrane fraction. Furthermore, we found that d(3)-GBB was taken up by rOctn2 in an Na(+)-dependent manner with K(m) value of 13 microM. The apparent K(m) value for d(3)-GBB transport in freshly isolated rat hepatocytes was 9 microM. d(3)-GBB uptake by the rat hepatocytes was inhibited by gamma-aminobutyric acid (GABA) to 30% of the control, whereas it was inhibited by carnitine to 62% of the control, even at 500 microM. Furthermore, d(3)-GBB uptake by rat hepatocytes was decreased by 45% with rat Gat2 (Slc6A13, a major liver GABA transporter) silenced by the microRNA method. Accordingly, the present study clearly demonstrates that GBB is taken up by hepatocytes for carnitine biosynthesis not only via Octn2 but also via the GABA transporter, possibly Gat2.

Effect of carnitine deprivation on carnitine homeostasis and energy metabolism in mice with systemic carnitine deficiency

BACKGROUND/AIMS

Juvenile visceral steatosis (jvs-/-) mice lack the activity of the carnitine transporter OCTN2 and are dependent on carnitine substitution. The effects of carnitine deprivation on carnitine homeostasis and energy metabolism are not known in jvs-/- mice.

METHODS

jvs-/- mice were studied 3, 6 and 10 days after carnitine deprivation, and compared to jvs-/- mice substituted with carnitine, wild-type (jvs+/+) and jvs+/- mice. Carnitine concentrations were assessed radioenzymatically.

RESULTS

Compared to wild-type mice, carnitine-treated jvs-/- mice had decreased plasma beta-hydroxybutyrate levels and showed hepatic fat accumulation. The carnitine levels in plasma, liver and skeletal muscle were decreased by 58, 16 and 17%, respectively. After ten days of carnitine deprivation, the plasma carnitine concentration had fallen by 87% (to 2.3 mumol/l) and the tissue carnitine levels by approximately 50% compared to carnitine-treated jvs-/- mice. Carnitine deprivation was associated with a further drop in plasma beta-hydroxybutyrate and increased hepatic fat. Skeletal muscle glycogen stores decreased and lactate levels increased with carnitine deprivation, whereas tissue ATP levels were maintained.

CONCLUSIONS

In jvs-/- mice, tissue carnitine stores are more resistant than carnitine plasma concentrations to carnitine deprivation. Metabolic changes (liver steatosis and loss of muscle glycogen stores) appear also early after carnitine deprivation.

Metabolomics identifies perturbations in human disorders of propionate metabolism

BACKGROUND

We applied untargeted mass spectrometry-based metabolomics to the diseases methylmalonic acidemia (MMA) and propionic acidemia (PA).

METHODS

We used a screening platform that used untargeted, mass-based metabolomics of methanol-extracted plasma to find significantly different molecular features in human plasma samples from MMA and PA patients and from healthy individuals. Capillary reverse phase liquid chromatography (4 microL/min) was interfaced to a TOF mass spectrometer, and data were processed using nonlinear alignment software (XCMS) and an online database (METLIN) to find and identify metabolites differentially regulated in disease.

RESULTS

Of the approximately 3500 features measured, propionyl carnitine was easily identified as the best biomarker of disease (P value 1.3 x 10(-18)), demonstrating the proof-of-concept use of untargeted metabolomics in clinical chemistry discovery. Five additional acylcarnitine metabolites showed significant differentiation between plasma from patients and healthy individuals, and gamma-butyrobetaine was highly increased in a subset of patients. Two acylcarnitine metabolites and numerous unidentified species differentiate MMA and PA. Many metabolites that do not appear in any public database, and that remain unidentified, varied significantly between normal, MMA, and PA, underscoring the complex downstream metabolic effects resulting from the defect in a single enzyme.

CONCLUSIONS

This proof-of-concept study demonstrates that metabolomics can expand the range of metabolites associated with human disease and shows that this method may be useful for disease diagnosis and patient clinical evaluation.

Effect of l-carnitine on the kinetics of carnitine, acylcarnitines and butyrobetaine in long-term haemodialysis

BACKGROUND

The current study was performed to investigate the kinetics of carnitine, individual acylcarnitines and butyrobetaine in patients on haemodialysis.

METHODS

Eight stable long-term haemodialysis patients were studied under basal conditions (no carnitine supplementation) and 3 weeks after intravenous supplementation with l-carnitine (10 or 20 mg/kg body weight) after each haemodialysis session. The kinetic studies included serial determinations of carnitine and metabolites just before, during or between haemodialysis sessions. Analysis was performed by liquid chromatography-tandem mass spectrometry.

RESULTS

Before haemodialysis, the plasma concentrations were (micromol/l) 15.1+/-0.6 (mean+/-SEM) for carnitine, 5.9+/-0.7 for acetylcarnitine, 0.66+/-0.04 for propionylcarnitine and 0.98+/-0.08 for butyrobetaine (basal conditions) or 142+/-23 for carnitine, 69+/-12 for acetylcarnitine, 6.0+/-1.1 for propionylcarnitine and 2.6+/-0.3 for butyrobetaine (carnitine 20 mg/kg). During haemodialysis, the plasma concentrations dropped by approximately 80% for all compounds determined, with extraction coefficients ranging from 0.65 to 0.86. In patients supplemented with 20 mg/kg carnitine, the amount of carnitine removed by haemodialysis equalled 42% of the dose administered, consisting of 2.08 mmol carnitine, 1.03 mmol acetylcarnitine and 0.051 mmol propionylcarnitine. Between the haemodialysis sessions, carnitine, acylcarnitines and butyrobetaine reached apparent steady-state concentrations within 1 day both under basal conditions and after supplementation.

CONCLUSIONS

Patients on haemodialysis have reduced carnitine, acylcarnitine and butyrobetaine plasma levels, which can be increased by supplementing carnitine. Propionylcarnitine, an important constituent of the acylcarnitine pool, can be removed by haemodialysis. Removal of potentially toxic acyl-groups may represent a mechanism for a beneficial effect of carnitine in these patients.

Supplementation of L-carnitine in athletes: does it make sense?

Studies in athletes have shown that carnitine supplementation may foster exercise performance. As reported in the majority of studies, an increase in maximal oxygen consumption and a lowering of the respiratory quotient indicate that dietary carnitine has the potential to stimulate lipid metabolism. Treatment with L-carnitine also has been shown to induce a significant postexercise decrease in plasma lactate, which is formed and used continuously under fully aerobic conditions. Data from preliminary studies have indicated that L-carnitine supplementation can attenuate the deleterious effects of hypoxic training and speed up recovery from exercise stress. Recent data have indicated that L-carnitine plays a decisive role in the prevention of cellular damage and favorably affects recovery from exercise stress. Uptake of L-carnitine by blood cells may induce at least three mechanisms: 1) stimulation of hematopoiesis, 2) a dose-dependent inhibition of collagen-induced platelet aggregation, and 3) the prevention of programmed cell death in immune cells. As recently shown, carnitine has direct effects in regulation of gene expression (i.e., carnitine-acyltransferases) and may also exert effects via modulating intracellular fatty acid concentration. Thus there is evidence for a beneficial effect of L-carnitine supplementation in training, competition, and recovery from strenuous exercise and in regenerative athletics.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Identification and tissue distribution of two differentially spliced variants of the rat carnitine transporter OCTN2

In this paper we show that the only known Na(+) dependent transporter of carnitine in mammals, organic cation transporter number 2 (OCTN2), is subject to differential splicing. Cloning of OCTN2 in different rat tissues identified two splicing variants. We have developed a real time quantitative polymerase chain reaction method for quantification of these splice variants. Both splice variants could be detected in all tissues examined with a relative abundance of 0.1-1% of the full length transcript. We also draw attention to the previously described mutations in clinical examples of primary carnitine deficiency in humans where the described mutations appear to be those of a splicing or mis-splicing event.

Na(+)-coupled transport of L-carnitine via high-affinity carnitine transporter OCTN2 and its subcellular localization in kidney

The mechanism of Na(+)-dependent transport of L-carnitine via the carnitine/organic cation transporter OCTN2 and the subcellular localization of OCTN2 in kidney were studied. Using plasma membrane vesicles prepared from HEK293 cells that were stably transfected with human OCTN2, transport of L-carnitine via human OCTN2 was characterized. Uptake of L-[(3)H]carnitine by the OCTN2-expressing membrane vesicles was significantly increased in the presence of an inwardly directed Na(+) gradient, with an overshoot, while such transient uphill transport was not observed in membrane vesicles from cells that were mock transfected with expression vector pcDNA3 alone. The uptake of L-[(3)H]carnitine was specifically dependent on Na(+) and the osmolarity effect showed that Na(+) significantly influenced the transport rather than the binding. Changes of inorganic anions in the extravesicular medium and of membrane potential by valinomycin altered the initial uptake activity of L-carnitine by OCTN2. In addition, the fluxes of L-carnitine and Na(+) were coupled with 1:1 stoichiometry. Accordingly, it was clarified that Na(+) is coupled with flux of L-carnitine and the flux is an electrogenic process. Furthermore, OCTN2 was localized on the apical membrane of renal tubular epithelial cells. These results clarified that OCTN2 is important for the concentrative reabsorption of L-carnitine after glomerular filtration in the kidney.

Effect of gamma-butyrobetaine on fatty liver in juvenile visceral steatosis mice

We pharmacokinetically examined the effect of gamma-butyrobetaine, a precursor of L-carnitine, on the change of fatty acid metabolism in juvenile visceral steatosis (JVS) mice, which have systemic L-carnitine deficiency due to lack of L-carnitine transporter activity. The concentrations of total free fatty acid (FFA), palmitic acid and stearic acid in the liver of JVS mice were significantly higher than those in wild-type mice. After intravenous administration of gamma-butyrobetaine (50 mg kg(-1)), the concentration of L-carnitine in the plasma of JVS mice reached about twice that of the control level and levels in the brain, liver and kidney were also significantly increased, whereas those in wild-type mice hardly changed. Although the plasma concentrations of FFA in both types of mice were unchanged after administration of gamma-butyrobetaine, the concentrations of palmitic acid and stearic acid were significantly decreased. In particular, the liver concentration of FFA in JVS mice was decreased to the wild-type control level, accompanied by significant decreases in long-chain fatty acids, palmitic acid and stearic acid, whereas those in wild-type mice were not changed. These results suggest that gamma-butyrobetaine can be taken up into organs, including the liver, of JVS mice, and transformed to L-carnitine. Consequently, administration of gamma-butyrobetaine may be more useful than that of L-carnitine itself for treatment of primary deficiency of carnitine due to a functional defect of the carnitine transporter.